As you probably know, clinical trials are studies performed on human subjects. Conducting trials and applying the results to large populations requires specific methods. The methods linked to clinical trials are presented in Unit 1 “Medical research an development”. Clinical trials are conducted when experimentation on humans is absolutely necessary to answer a specific question and must be carried out in accordance with ethical principles. Ethical questions linked to clinical trials are discussed in Unit “Ethics”. Some basic aspects of statistics and their applications to questions related to clinical trials are presented in this unit.